The term “battery” originally meant a plurality of electrochemical cells connected in series in a housing. However, even single electrochemical cells are nowadays frequently referred to as a battery. During discharge of an electrochemical cell, an energy-supplying chemical reaction made up of two electrically coupled, but spatially separated part reactions takes place. A part reaction which takes place at a relatively low redox potential proceeds at the negative electrode and a part reaction takes place at a relatively high redox potential at the positive electrode. During discharge, electrons are liberated by an oxidation process at the negative electrode, resulting in an electron current flowing via an external load to the positive electrode which takes up a corresponding quantity of electrons. Thus, a reduction process takes place at the positive electrode. At the same time, an ion current corresponding to the electrode reaction flows within the cell. This ion current is ensured by an ionically conductive electrolyte. In secondary cells and batteries, this discharging reaction is reversible, i.e., it is possible to reverse the transformation of chemical energy into electric energy that occurred during discharge. Where the terms “anode” and “cathode” are used in this context, the electrodes are generally named according to their function during discharging. The negative electrode in such cells is thus the anode, and the positive electrode is the cathode.
Each electrode comprises at least one electrode active material and electrochemically inactive components. An electrode active material experiences a chemical transformation during charging and discharging, in particular an oxidation or reduction (the above-mentioned oxidation and reduction processes). In contrast, electrochemically inactive components are essentially unaffected during charging and discharging. Examples of electrochemically inactive components of an electrode are electrode binders, current collectors, power outlet leads and conductivity-improving additives. Electrons are supplied to or conducted away from the electrodes by power outlet leads. Electrode binders ensure the mechanical stability of the electrodes and contacting of the particles of electrochemically active material with one another and with the power outlet lead. Conductivity-improving additives contribute to an improved electric connection between the electrochemically active particles and the power outlet lead.
Lithium-ion batteries are the most frequently used secondary batteries for portable electronic devices. The mechanism for charge storage is based on the intercalation/deintercalation of Li-ions in usually metal oxides (electrode active material on the cathode side) and carbon (electrode active material on the anode side). Lithium-ion batteries exhibit good energy density and cycle stability, but moderate capacities or capacity retention at higher current densities because of the slow rates of the electrode reactions, resulting in low power densities.
In contrast, supercapacitors (formerly electric double-layer capacitors) can provide high pulsed currents at high power densities. However, the capacity of double-layer capacitors is limited according to the nature of a capacitor. Furthermore, like all capacitors, supercapacitors have a falling voltage characteristic.
Battery cells utilizing organic compounds as electrode active materials are capable of showing higher power densities and also higher gravimetric energy densities than lithium-ion batteries. One example is the utilization of reversible oxidation/reduction-reactions of compounds containing stable radicals such as nitroxide radicals. For example, EP 2 025 689 A1 discloses the use of nitroxide radical containing polymers as electrode active materials in secondary batteries which show very good capacity retention at higher current rates (high power density) and cyclic stability.
However, many organic electrode materials such as, for example, conducting polymers or also some polyradicals start to degrade when cycled to and held at higher potentials, for example, at potentials close to or above 4 V vs. Li/Li+.
Compounds containing N,N,N′,N′-tetrasubstituted-1,4-phenylenediamine units are utilized in optoelectronic devices such as organic light emitting devices (OLED) or electrochromic devices. The use as electrochromic material has been described, for example, in Liou, G.; Chang, C. Macromolecules 2008, 41, 1667-1674. The preparation of polyamides containing N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units is described. As another example, U.S. Pat. No. 8,304,514 A discloses a polyfluorene compound containing N,N,N′,N′-tetraphenyl-p-phenylenediamine units and its utilization as electrochromic material.
The use of poly(3,4-ethylenedioxythiophene) containing N,N,N′,N′-tetraalkylated-1,4-phenylenediamine units as electrical energy storage material has been described in Conte, S.; Rodriguez-Calero, G. G.; Burkhardt, S. E.; Lowe, M. A.; Abruna, H. D. RSC Advances 2013, 3, 1957-1964. The use of a polymer containing N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units as cathode material for lithium ion batteries has been described in Chang Su; Fang Yang; LvLv Ji; Lihuan Xu; Cheng Zhang, J. Mater. Chem. A 2014, 2, 20083-20088.
It has been a major goal to manufacture Li-ion batteries with metal oxide electrodes having good capacity retention at high current densities or high power densities. On the other hand electric double layer capacitors show high power densities, but low energy densities.